Dioxin (PCDD) is an abbreviated name for poly chloro dibenzo-p-dioxin, which is one type of organic chlorine compound. Poly chloro dibenzo-furan (PCDF) is known which is similar to the dioxin.
In particular, 2,3,7,8-tetra chloro dibenzo-p-dioxin (2,3,7,8-T4CDD), which belongs to tetra chloro dibenzo-p-dioxins (T4CDDs), or tetrachlorides of dioxins (PCDDs) and has chlorine in the 2-, 3-, 7-, and 8-positions, is the highest in toxicity.
The 2,3,7,8-tetra chloro dibenzo-p-dioxin is generated as a by-product in the production of trichloro phenol, 2,4,5-trichloro phenoxyacetic acid, and also generated as chloridization of dibenzo-p-dioxin. The melting point is between 306° C. and 307° C.
PCB (polychlorinated biphenyl) is known, which is another organic chlorine compound harmful to humans. PCB high in chemical stability, insulation properties, incombustibility and adherence, has been used as insulating oil for transformers and capacitors installed in electrical equipment at power plants, railways and buildings. However, since PCB contains coplanar PCBs which are toxic as with dioxin, the substance has been prohibited from production, distribution and new applications on the basis of laws developed in 1974.
Methods for treating PCBs include, for example, an incineration treatment method for heat-treating PCBs at high temperatures, a dechlorination decomposition method for subjecting PCBs to dechlorination, a supercritical hydroxylation decomposition method for using supercritical water to decompose PCBs into carbon dioxide and water. Of these methods, with the incineration treatment method, there is a concern that dioxins may be synthesized when gases generated in heat-treating PCBs are cooled.
Then, conventional technologies are known, for example, those described in the following Patent Documents 1 and 2.
Patent Document 1 discloses a method in which gas exhausted from cement production facility is supplied to a dust collector for collecting dust including organic chlorine compounds, and at least some of the thus collected dust is fed into a high temperature part of the cement production facility, which is kept at temperatures exceeding 800° C. Since dioxins undergo thermal decomposition at about 800° C., this method can be used to effectively decompose dioxins. Gas exhausted from cement production facility includes, for example, exhaust gas from a raw material dryer (a raw material processing area) for drying cement raw materials, and exhaust gas from a raw material mill (a raw material processing area) for crushing cement raw materials.
Further, evaporated organic chlorine compounds are contained to some extent also in dust-removed gas which is exhausted from a dust collector and released into the atmosphere from a gas duct. Patent Document 1 has disclosed a method for reducing dioxin concentrations in dust-removed gas as a preventive measure thereof. In other words, exhaust gas is drawn out from a place at which the temperature is kept from 30° C. to 400° C., (a low temperature part) in cement production facility, and the thus drawn out exhaust gas is supplied to the dust collector. Exhaust gas guided from the low temperature part is higher in concentration of organic chlorine compounds (low-temperature concentration) compared with exhaust gas from a high temperature part of the cement production facility. Therefore, this method is used to collect and remove dust in which organic chlorine compounds are concentrated, thus making it possible to reduce the concentration of dioxins in the dust-removed gas.
Patent Document 2 discloses a method for decomposing PCBs, while generation of dioxins is prevented. In other words, PCB-containing materials delivered to a cement plant from the outside are fed into a rotary kiln at which the PCB-containing materials are heated by using heat (1000° C. or higher) generated on calcination of a cement clinker, thereby effecting thermal decomposition of PCBs. Then, after exhaust gas generated on a thermal decomposition is guided from the outside of the rotary kiln, the gas is cooled rapidly at a speed of 20° C./sec or higher. The exhaust gas is rapidly cooled at 20° C./sec or higher, by which the gas passes in a short time through a temperature range at which dioxins are synthesized in an increased quantity. Therefore, this method is used to decompose PCBs, while generation of dioxins is prevented.    PATENT DOCUMENT 1: Japanese Unexamined Patent Application, First Publication No. 2004-244308    PATENT DOCUMENT 2: Japanese Unexamined Patent Application, First Publication No. 2002-147722